氢还原下铁钼催化剂的表面性质

用XPS对H_2还原下的铁钼催化剂的表面性质进行了研究。实验表明:H_2还原使表面钼离子由Mo~(6+)还原为Mo~(5+)和Mo~(4+),并与Fe~(3+)作用氧化为Mo~(6+);Fe~(3+)则由于与Mo~(5+)和Mo~(4+)作用以及H_2还原而变为Fe~(2+),从而形成了一个没有O_2存在,仅有H_2作用下的Mars-Krevelen氧化还原循环过程。在催化剂表面上生成的FeMoO_4,其Mo~(6+)的3d3/2和3d5/2的结合能值比Fe_2(MoO_4)_3中的Mo~(6+)的3d能级结合能值低。Mo(或Mo和Fe)离子键合的O~(2-)与H_2作用生成的Mo(Fe)-OH,其Os结合能的测定值低于键合于Mo(Fe)离子中的O~(2-)的Os结合能值,并且该结合能峰由于-OH间的相互作用结合成的水的脱附而消失。此外.给出了还原过程中H_2与催化剂表面O~(2-)相互作用的反应图式。

XPS STUDY OF SURFACE PROPERTIES OF IRON-MOLYBDENUM CATALYST IN HYDROGEN REDUCTION

The surface properties of iron-molybdenum catalyst in H2 reduction have been studied with X-ray photoelectron spectroscopy. The results showed that Mo6 + ions on the sample surface were reduced to Mo5 + and Mo4 + , and then were oxidized to Mo6+ once more by the interaction with Fe3+ ions. Meanwhile, the Fe3 + ions, due to the interaction with Mo5+ and Mo4+ and also due to the reduction by H2, were reduced to Fe2+. Thence, it exhibited a picture of the cycle of Mars-Krevelen redox mechanism for Fe-Mo-O catalyst in the selective oxidation, in which the oxygen was absent and only the hydrogen existed. It was found that the binding energy(b.e.) of 3d3/2 and 3d5/2 of Mo6+ (235.1 and 232.0eV) of FeMoO4 existed on the catalyst surface were lower than those in Fe2(MoO4)3,. The O2- species bonded to the Mo (or Mo and Fe) species interacted with hydrogen to produce Mo(Fe)-OH on 1he surface, and their b.e. of Ols was 528.4eV, which was somewhat lower than that of O2-(530.0eV)in lattice. When the reduction was continued, due to the interaction of the hydro-xyl radicals of Mo(Fe)-OH, water was formed, and the peak of Ols of Mo(Fe)-OH disappeared when the water desorbed. In addition, the reaction pattern of the interaction between hydrogen and O2- on the catalyst surface was given.